Composite particles contain organic and inorganic domains, for example an organic polymer matrix and inorganic particles fixed thereto, and typically have a diameter of 4 to 5000 nm.
In common processes for producing composite particles, organic monomers are polymerized by means of emulsion polymerization in aqueous dispersions of inorganic particles, which anchors organic domains on the surface of the inorganic particles, as described, for example, in Dong-Ming Qi, J. of Applied Polym. Sci., 2006, Vol. 99, pages 3425 to 3432; Tsutomu Mizutani, J. of Applied Polym. Sci., 2006, Vol. 99, pages 659 to 669; Frank Bauer, Macromol. Mater. Eng., 2006291, pages 493 to 498, or in DE-A 10 2004 010 155, U.S. Pat. No. 3,544,500, U.S. Pat. No. 4,421,660 or WO-A 2006/072464.
However, the fixing of the organic and inorganic domains of the composite particles and the provision of stable composite particles present problems. This is because the inorganic particles or starting materials thereof and the organic monomers or the organic polymer matrices usually have different polarities and tend to separate from one another or agglomerate with one another.
If such an agglomeration occurs before or during the production of the composite particles, for example, the agglomerated inorganic particles are encapsulated by the organic polymer matrix in the course of polymerization of the organic monomers, such that there is no homogeneous fixing of inorganic particles on the organic polymer matrix, and hence there is ultimately no formation of chemically homogeneous composite particles formed from organic and inorganic domains. Corresponding mixtures are not in the form of colloidal primary particles in solvents. It may even be the case that the inorganic particles and the organic polymer matrix are present alongside one another as a blend.
However, there may also be agglomeration of the inorganic or organic domains in composite particles formed homogeneously from inorganic and organic domains, which lead to inadequate storage stability, especially of dissolved, emulsified or dispersed composite particles, and is manifested by gelation or spot formation. Especially at relatively high temperatures, for example from 40° C., such agglomeration occurs. Aqueous dispersions of particles formed exclusively from inorganic units, for example colloidal silica or organopolysiloxanes, likewise tend to agglomerate at relatively high temperatures, for example at 70° C., or even at room temperature.
Agglomerated composite particles no longer have the desired performance properties or are completely unusable. In order to provide, for example, stable aqueous dispersions of composite particles, typically emulsifiers, protective colloids or specific additives are added as stabilizers, for example the hydroxyl-containing alkylamino compounds recommended in DE-A 10 2004 010 155.